The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In general, an oxygen sensor is mounted at a predetermined position at an exhaust manifold and detects an oxygen amount of the exhaust, and the oxygen sensor provides the oxygen amount information to an engine control unit as feedback information.
Therefore, the engine control unit may determine whether an air/fuel ratio is lean or rich by receiving information from the oxygen sensor, and may control the engine so as to perform combustion with a theoretical air-fuel ratio.
A typical oxygen sensor includes a first oxygen sensor which is installed at a front end of a catalyst device and a second oxygen sensor which is installed at a rear end of the catalyst device, such that the catalyst device is disposed between the first oxygen sensor and the second oxygen sensor.
The first oxygen sensor provides the feedback information about leanness or richness of the air-fuel ratio to the engine control unit. The second sensor senses concentration of oxygen included in the exhaust gas, which is exhausted into the atmosphere after the exhaust gas is purified by the catalyst device, and provides the feedback information to the engine control unit. The first oxygen sensor and the second oxygen sensor may be provided for achieving emission control.
The first oxygen sensor is activated when it is heated by the exhaust gas which is exhausted from the engine and by a heater which is provided thereto. The reason why the oxygen sensor is heated is because a material of a sensing part of the oxygen sensor is activated and operates well when a temperature is over 700° C.
Therefore, if the oxygen sensor is not heated properly, it may not sense and provide information of the leanness or richness of the air-fuel ratio appropriately as a feedback signal. Thus, the engine does not perform combustion at a theoretical air-fuel ratio and it may have an adverse effect on the emission control.
To activate the oxygen sensor when the engine starts, the heater which is disposed in the oxygen sensor is operated. And after the engine starts, as time passes, when the temperature of the exhaust gas is over predetermined temperature, the heater may be controlled by applying a predetermined map value.
Accordingly, overheating of the oxygen sensor is prevented and an appropriate temperature may be maintained.
Meanwhile, the oxygen sensor includes a sensor element, a heater, and a protection tube. The sensor element and the heater are disposed in the protection tube, and the sensor element is placed at a portion where the exhaust gas flows. Therefore, a substance of combustion material of the exhaust gas may adhere to the sensor element, and responsiveness of the sensor element may thereby be delayed or deteriorated.
Particularly, if the temperature in the protection tube is lower than that of the exhaust gas, by an ideal gas equation (PV=nRT), a pressure in the protection tube is relatively lower than at an outer side. Thus, the exhaust gas quickly flows into the protection tube. At this time, if the engine starts, the exhaust gas which has flowed into the protection tube is stabilized, and the combustion material of the exhaust gas may contaminate the sensor element.